In a PON system, communication is performed while achieving synchronization between the OLT and the ONU so that pieces of uplink data transmitted from the ONUs do not collide with each other. The OLT plans to grant a transmission permission to each ONU so that the pieces of uplink data do not collide with each other. At this time, the OLT considers delay due to a distance between respective ONUs. Therefore, the OLT performs measurement of a roundtrip time between respective ONUs and keep-alive communication. However, there are fluctuations in a transmission line such as jitter and wander in transfer by an optical fiber, and thus measurement needs to be performed cyclically.
Meanwhile, data communication is not performed constantly. For example, data communication is not performed at all during the night. However, measurement of the roundtrip time and transmission and reception of a keep-alive signal are performed cyclically regardless of the presence of data communication. It is a waste of power to maintain the ONU in a transmissible state at all times for measurement of the roundtrip time, even if data communication is not performed. Accordingly, a technique for causing the ONU to shift to a power saving state intermittently by requesting a shift to the power saving state from the ONU has been studied.
Furthermore, conventionally, there is a PON system in which an OLT generates a power-supply control signal of an ONU that does not require communication to control a power supply of a transmission circuit and a reception circuit of the ONU (Patent Literature 1). In this PON system, because the OLT remotely controls on/off of the power supply of the ONU, power consumption of the entire system can be reduced.